<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <form>
        <input type="text" id="username">
        <input type="password" id="pwd">
        <input type="submit" value="注册" id="sub">
        </form>
</body>
<script>
    /* window.onload = function(){
        console.log("页面加载完成");
    }
    window.onunload=function(){
        alert("页面关闭"); */

    // }
    var f1 = document.getElementById("f1");
    var username = document.getElementById("username");
    var pwd = document.getElementById("pwd");
    var sub = document.getElementById("sub");
    username.onblur= function(){
        var val = username.value;
        if (val.length<3) {
            alert("用户名过短");
            
        }

    }
    f1.onsubmit = function(){
        alert("");
    }
    </script>
</html>